This invention relates to semiconductor devices and particularly to an improved junction field effect transistor (JFET).
A conventional JFET is a three-terminal semiconductor device in which a current flows substantially parallel to the top surface of the semiconductor chip and the flow is controlled by a vertical electric field, as shown in FIG. 1a, 1b, and 1c. It can be used as a current switch or a signal amplifier.
JFETs are known as unipolar transistors because the current is transported by carriers of one polarity, namely, the majority carriers. This is in contrast with bipolar junction transistor, in which both majority-and-minority-carrier currents are important.
A typical n-channel JFET fabricated by the standard planar process is shown in FIG. 1. FIG. 1a depicts an n-channel JFET of which the channel is a part of an epitaxial semiconductor layer. FIG. 1b depicts another n-channel JFET of which the channel is formed with a double-diffusion technique in a semiconductor substrate. FIG. 1c is a schematic representation of the JFETs.
As depicted in FIGS. 1a and 1b, the body of the JFET comprises a lightly doped n-type channel sandwiched between two heavily doped p+-gate regions. In FIG. 1a, the lower p− region is the substrate, and the upper p+ region is a portion of the silicon epitaxial layer into which boron atoms are diffused. The two p+ regions may be connected either internally or externally to form the gate terminal. Ohmic contacts are attached to the two ends of the channel to form the drain and source terminals through which the channel current flows. Alternatively, as illustrated in FIG. 1b, a JFET may be fabricated by a double-diffusion technique where the channel is formed by diffusing n-type dopant into the substrate. In both cases, the channel and the gate regions run substantially parallel the top surface of the substrate, so does the current flow in the channel. The operation of a JFET in a switching mode is briefly described in the next paragraph.
When the gate voltage is close to the channel potential, mobile charge carriers flow freely in the channel region between the source and the drain terminals. This is the ON state. To reach the OFF state, one may apply a reverse-biasing voltage to the gate terminals. The reverse bias voltage applied across the gate-channel junctions “pinches off” the channel by depleting mobile charge carriers from the channel and produces space-charge regions that extend across the entire width channel.
With the gate voltage set between ON and OFF levels, the effective cross-sectional area of the channel can be varied and so can the channel resistance to the current flow. Thus the current flow between the source and the drain is modulated by the gate voltage.
An important figure of merit of a JFET is its cutoff frequency (fco), which can be represented mathematically as follows:fco≦qa 2 μnNd/(4πk∈oL2),where q is the electric charge of the charge carriers, a is the channel width, μn is the mobility of the charge carriers, Nd is the doping concentration in the channel, k and ∈o are the dielectric constant of the semiconductor material and the electrical permittivity of the free space respectively, and L is the channel length.
Another important figure of merit of a JFET is the noise figure. One dominant noise source in a transistor is the interaction of the mobile charge carriers and crystal imperfections in the device. This gives rise to the 1/f noise spectrum.
This invention provides a JFET device that has superior fco and 1/f performance over conventional JFETs and a process of making the device.